In connection with electronic pictures like TV pictures, noise is often a problem. The noise with which one is most familiar, is what can be called thermal noise--or random noise. It is possible to achieve clearer and cleaner pictures if this type of noise is reduced somewhat.
Attempts have therefore been made to find methods for a reduction of this noise in the temporal domain by using various filtering techniques, which will often be based upon a type of recursive filter means. Examples of methods of such a type of noise reduction can be found in e.g. British patent application No. 2,020,941, U.S. Pat. No. 4,639,784 and German patent DE 33 11 898. All these patent publications are based upon a temporal picture filtering, so that the filtered picture will be less afflicted with noise, and hence present a more pleasant appearance, and simultaneously the signal time variation will not become veiled.
Lately one has started to use digital compression techniques to reduce bit consumption when representing single pictures or picture sequences (live pictures). These are e.g. techniques used in picture telephony. The coding technique itself is standardized by the international telecommunication organization CCITT. Live pictures are encoded in such a manner that they do not need a higher network transmission capacity than in the case of telephony. In order to succeed in such a task, the live picture must be compressed to such a degree that also visible distortion is introduced in the picture. This distortion has the effect that the picture looks a little less pleasant. In order to remedy this effect, it is possible to apply a subsequent filtering of the picture to give the picture a more pleasant appearance.
Thus, a situation exists where a great deal of knowledge exists regarding the generated noise. If one for example utilizes blocks when encoding pictures, one knows as a starting point that the block patterns will easily become visible and disfiguring. This information regarding the type of noise to be removed, becomes handy when the task is to construct a subsequent filter, i.e. a cosmetics filter. The goals to be achieved by means of such a filter can be summarized in this manner:
A removal of the noise generated in the encoding process, so that the result becomes subjectively pleasant to the eye,
taking care that scars from possible block magnitudes in the encoding process will not become visible, and
effecting the filtering in such a manner that no veiling or further lack of definition are introduced in the picture.
There exists a clear need for such cosmetics filters. One version of such a filter is described in "Motion video coding for visual telephony" by Ronald Plompen, PTT Research Neher Laboratories. This filter satisfies in general the requirements stated above, however the filter is rather demanding as to calculations. The described filter is intended used for block based encoding methods, and this assumes that the signal power is calculated for each respective block. This requires a lot of extra calculating power.